It is desired a unit injector for a high pressure injection of a diesel engine in order to improve an air pollution due to the diesel engine and reduce a fuel consumption. As an example, there has been suggested a structure described in Japanese Utility Model Unexamined Publication No. 2-145659.
FIG. 3 is a cross sectional view of a fuel injector 10a (a unit injector) for a high pressure injection suggested in the publication mentioned above. A plunger 11 is closely inserted in a plunger hole 14 provided in a housing 13 in such a manner as to protrude and freely slide in a vertical direction. The plunger 11 performs a vertical reciprocating motion by a return spring 12 and a drive force from a cam shaft (not shown). A pressure chamber 15 is formed in a lower end portion of the plunger 11. An annular groove 20 for returning a fuel (hereinafter, refer to as an annular groove 20) is provided on an inner peripheral surface of the plunger hole 14, and is connected to a fuel tank (not shown) via a narrow drain passage 21. A fuel injection nozzle 17 provided with a needle valve 16 in an inner portion thereof is mounted to a lower end portion of the housing 13, and the needle valve 16 is urged downward by a spring 18 so as to close a fuel injection port 19. The pressure chamber 15 and the fuel injection port 19 are communicated with each other by a passage 19a.
A poppet valve 30 having an annular conical valve seat 31 and driven by a solenoid and a solenoid stator 32 operating the poppet valve 30 are provided in a side portion of the housing 13. A fuel storage 33 is provided in a lower portion of the poppet valve 30, and the fuel storage 33 and a drain oil storage 34 provided in an upper portion of the annular conical valve seat 31 are communicated with each other by a drain fuel passage 35 connected to the fuel tank (not shown). Further, the fuel storage 33 is communicated with the fuel tank (not shown) by a housing inside supply passage 36 provided in the housing 13, and is always supplied a low pressure fuel by the fuel supply pump. Still further, the lower portion of the annular conical valve seat 31 and the pressure chamber 15 are communicated with each other by a fuel supply passage 37.
Next, an operation will be described below.
Since the low pressure fuel is always supplied to the housing inside supply passage 36 of the housing 13 from the fuel supply pump, the fuel storage 33 and the drain oil storage 34 are filled with the low pressure fuel. When operating the poppet valve 30 by the solenoid stator 32 so as to open the annular conical valve seat 31, the low pressure fuel in the drain oil storage 34 is supplied within the pressure chamber 15 via the fuel supply passage 37.
Next, when closing the annular conical valve seat 31 so as to drive the plunger 11 downward through a drive row assembly due to a rotation of a cam, the fuel within the pressure chamber 15 becomes a high pressure. Further, when a pressure of the fuel injection port 19 reaches a predetermined pressure, the needle valve 16 is pressed down so as to inject a fuel within a combustion chamber of an engine (not shown). When the fuel within the pressure chamber 15 becomes a high pressure, the fuel is lifted up with passing through a gap between the plunger 11 and the plunger hole 14, and the fuel is returned to the fuel tank (not shown) from the annular groove 20 through the drain passage 21, however, is not leaked outward. In this case, a contact surface between the drive row assembly and the plunger 11 is lubricated by a lubricating oil.
However, in accordance with the structure mentioned above, the lubricating oil supplied to a portion between the drive row assembly driven by the cam and the plunger 11 is attached to an outer diameter portion of the plunger 11, is leaked downward from the gap between the plunger hole 14 and the plunger 11 when the plunger 11 descends within the plunger hole 14, and is discharged to the fuel tank via the drain passage 21 after entering into the annular groove 20. Accordingly, there are problems that the fuel tank is soiled in black by the lubricating oil and a lubricating oil consumption amount is increased.